1. Field of the Invention
The present invention relates to a technique for recording a halftone image representing a color original image by comparing the original image with a threshold matrix for each color component.
2. Description of the Background Art
Conventionally used has been an inkjet printer provided with a head having a plurality of outlets, where the head moves relatively to a printing paper and printing is performed by controlling ON/OFF of ejection of fine droplet of ink from each outlet. Since a grayscale image to be written (hereinafter, referred to as “original image”) has to be binarized in such a printer, a halftone image representing the original image is generated. In an AM (Amplitude Modulated) screening, gray levels are represented by changing the size of dots without changing the number of dots constituting the halftone image. In an FM (Frequency Modulated) screening, gray levels are represented by changing the number of dots of certain size, which are arranged appropriately (in a random fashion without extreme local difference in density).
In the inkjet printer, there is a case where a bleary line(s) (hereinafter, referred to as “streak unevenness”) extending in a scan direction appears in a printed halftone image because of variations in amounts of ejection of fine droplets from the plurality of outlets, variations in directions of ejection, or the like. Japanese Patent Application Laid-Open No. 2004-106248 (Document 1) discloses a method of reducing streak unevenness, where a density measurement part is provided in a printer, a density of a region corresponding to each outlet in a pattern with a uniform density level (as a setting) which is printed by a head (hereinafter, the density is referred to as “the print density in the outlet”) is acquired, and then each pixel value of an original image is modified on the basis of the print density in the outlet corresponding to the pixel value (i.e., the shading compensation is performed practically). In order to strictly acquire the print density in each outlet, the relationship of a recording resolution of the printer with regard to an arrangement direction of the outlets and a resolution of reading in the density measurement part (for example, a scanner) has to satisfy 1 to α (α is an integer which is 1 or more), however actually, even if such condition is not satisfied, a low-frequency unevenness can be roughly suppressed as long as the print density in the outlet is acquired with accuracy of a certain degree.
Japanese Patent Gazette No. 3414325 discloses a method of suppressing occurrence of streak unevenness which easily appears when printing is performed by recording only specific dots with a relatively small diameter in an inkjet printer. In the method, the specific dots and dots with a larger diameter than that of the specific dots are mixed at a meaningful rate in the printer. Also, Japanese Patent Application Laid-Open No. 2006-14271 discloses a method of suppressing graininess in a generated color halftone image, where threshold matrixes of color components are generated so that growing of halftone dot areas with increase of gray level in one color component shows different directional property from another color component.
In the case that dots are periodically arranged in the halftone image like in the AM screening, if it is tried to reduce streak unevenness by the method of Document 1, a pitch of dots in a pattern which is a halftone image for density measurement and a pitch of reading positions which is the reciprocal of resolution of scanner interfere and data of an image including interference unevenness of low frequency (i.e., the image including a beat pattern) is read out by the scanner. Thus, when each pixel value of the original image is modified on the basis of such image data, a beat pattern appears in the generated halftone image.
In the color halftone image, there is a problem that graininess easily appears in the image (that is to say, granularity of the image worsens) because of overlapping halftone images of a plurality of color components, and it is extremely difficult to achieve reduction of graininess and prevention of occurrence of streak unevenness and a beat pattern in the printed color halftone image.